There are a number of standardized descriptions for digital image data such as JPEG, GIF, BMP, EXIF, PCD, TIFF, etc. Every standard digital file format specifies a set of standard descriptions, such as image width, height, bits per pixel, number of layers, etc. A specific decoder is needed to decode image data that is encoded in each standard description. A standard specifies a set of standard descriptions such that all parties speak the same language and can communicate with each other. However, there is always a tradeoff between the standard complexity and the descriptive power of the standard. A simple standard such as PGM may only specify the image dimension and the magic number (a number of bytes indicating a PGM format), enough for decoding the raw data. While other standards, such as JPEG2000, may specify far more complicated descriptions and description schemes to enable more functionality. In practice, to limit the complexity of an image decoder, a standard with a limited number of descriptors is preferred.
Any image descriptors outside the scope of a digital image standard are called non-standard descriptions. It is not possible to describe an image for every foreseeable task by only using a standard description. For example, many standard image file formats do not specify if there are human faces in the image. This data may be useful for some applications. For another example, the date and time information is part of the standard image header of a TIFF image, but it is a non-standard description for a BMP image. Also digital audio can be embedded as standard description in EXIF image files, but it is non-standard for JPEG, GIF, TIFF, BMP, etc. It will be appreciated that non-standard descriptions can be very helpful for many task-dependent and user-dependent applications. There has been great effort on standardizing image description. The goal is to come up with a set of standard descriptions for a still image so that the image is exchangeable across a variety of media and systems. JPEG and EXIF are the typical examples. New file formats are invented to accommodate more standard descriptions in image headers, such as the metadata encoded by XML in EXIF.
The prior art fails to address how to use both standard and non-standard descriptions in a self-contained digital file for complete and enhanced image description and still make the non-standard descriptions compatible with the widely adopted standard image decoders. There is a need therefore for an improved image description method that can use both standard and non-standard descriptions in a self-contained digital file for complete and enhanced image description and still make the non-standard descriptions compatible with the widely adopted standard image decoders.